1. Technical Field of the Invention
This invention relates generally to wireless communication devices and more particularly to radio frequency transmitters used within such wireless communication systems.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is further known, the transmitter of a wireless communication device transmits RF signals that represent baseband processed data to the receiver of another wireless communication device directly or through an access point, or base station. The particular type of baseband processing used to prepare the data for radio frequency transmission and subsequent data recapture by the receiver is dependent upon the standard, or standards, being supported by the wireless communication devices and upon the received signal strength of the RF signals. For example, if the standard being supported is IEEE 802.11g, the baseband processing may include encoding data at 1 or 2 megabits-per-second using a direct sequence spread spectrum (DSSS) encoding protocol, a 5.5 or 11 megabits-per-second complimentary code keying (CCK) encoding protocol, or a 6, 9, 12, 18, 24, 36, 48, or 54 orthogonal frequency division multiplexing (OFDM) encoding protocol.
The particular encoding protocol selected is at least partially based on received signal strength indication (RSSI). In general, the weaker the signal strength of the received RF signal, the lower the data rate. As is known, the transmitter transmits RF signals at a fixed output power level (e.g., 10 dBm) such that the decrease in receive signal strength is primarily due to physical distance between the transmitter and receiver. Thus, the greater the distance, the weaker the received signal will be, requiring a lower data rate encoding protocol to be used. Further, when the received signals are weak, the data recovery circuitry within the receivers may have difficulty recapturing the data resulting in a retransmission of the data or loss of data, either of which reduces data throughput.
One solution for improving received signal strength is to raise the transmit power level such that the received signal strength would correspondingly increase. While this would help for weaker received signal strengths, it is overkill for stronger received signals and would unnecessarily consume additional power in the transmitter, which, for battery operated wireless communication devices is extremely detrimental.
Therefore, a need exists for a method and apparatus for adjusting transmit power levels, while maintaining the particular transmit power level constant, without increasing power consumption and further improving data throughput.